Adequate blood flow through the brain at all times is not only an unalterable prerequisite for normal cerebral function, but also an essential necessity for the survival of the individual in situations of impaired cardiocirculatory function. Even short-term interruptions of the cerebral blood flow can cause irreversible damage which either leads to loss of life or is followed by the most serious defective healing.
In medicine, monitoring of cerebral blood flow poses a particular problem in the case of unconscious patients, for example during general anaesthesia or with intensive-care patients on a respirator. In these patients, cerebral function must also be suppressed for reasons of pain avoidance, and thus there is no possibility of indirectly judging to what extent cerebral blood flow is adequate. The problem is particularly severe with patients suffering from cerebral diseases, such as for example the most severe cerebral concussion and cerebral tumour.
On the one hand in these cases, the brain tends to swell, whereby blood flow can be impeded. On the other hand, decoupling of cerebral blood flow from the metabolic requirements of the brain may occur whereby the accompanying expansion of blood vessels can cause cerebral swelling in itself. While in the one case therapeutic measures aiming at promoting cerebral blood flow would have to be undertaken, in the other case a reduction in cerebral blood flow would have to be aimed at.
It is therefore desirable to find not only a simple method, especially one that can be applied at the bedside, for measuring cerebral blood flow and intracerebral blood volume, but also one that offers simple monitoring for effectiveness and dosage rate when testing newly developed medicaments for promoting or reducing cerebral blood flow.
So far, only relatively expensive processes have been available for measuring cerebral blood flow in patients. As a rule, these are indicator dilution processes in which foreign gases are used as indicators. In a technique that has been used for a long time (Kety-Schmid technique) saturation of the blood and the brain with laughing gas or argon takes place, whereby the respective gas is enriched in the respiratory air. The analysis process takes place concurrently by means of several blood samples in the arterial and in the cerebrovenous blood for which firstly an arterial catheter and secondly a catheter in the "bulbus venae jugularis" is required. The latter is inserted retrogradely by way of a jugular vein (vena jugularis interna), to the level of the base of the skull.
With a different method, radioactively marked xenon is used which is injected as a bolus in aqueous solution. With this technique, gas indication is by means of extra cranial detectors. Due to their associated costs as well as due to the equipment required, both these methods are feasible during operations or with intensive-care patients only for scientific purposes; they are unsuitable for the demands of clinical routine. In the Kety-Schmid process, the blood samples must be processed through extensive laboratory analysis, so that the result is usually available only hours later. Apart from the high cost, the main disadvantage of the xenon technique is the associated radiation exposure for both patient and personnel.
From Journal of Applied Physiology, vol. 64, nr. 3, March 1988, pages 1210-1216, Bock et al.: "Thermal recovery after passage of pulmonary circulation assessed by deconvolution" it is known to determine the cardiac stroke volume and the extravascular thermal lung volume. To this effect, a double indicator of heat/cold on the one hand, and dye on the other hand, the quantity of which must be precisely known, is injected into the bloodstream. Thereafter, the indicator dilution curves are measured and subsequently the areas below the curves are determined. The ratio of indicator quantity to area below the curves subsequently reveals the blood flow in absolute units, e.g. in milliliters per minute.
The indicator heat/cold serves to determine the blood flow "cardiac stroke volume". The extravascular thermal lung volume is determined from the product of cardiac stroke volume and mean transit time, whereby the intravascular lung volume is deducted, in which the difference between mean transit time of the indicator heat/cold and the indicator dye is constituted.